The necessity to measure the density of a liquid is widespread throughout the processing, mining and petro-chemical industries. Many systems have been developed to try to obtain an accurate and continuous measurement of a constantly changing weight of a liquid. If the liquid is stationary in the measuring instrument or of a particular consistency then deposits can form on the inner walls causing a build-up of material and an eventual erroneous measurement. Some instruments measure a constant flow of liquid therethrough, however many liquids, particularly those used in oil well drilling, will still form a coating or build-up within the instrument.
Many instruments employ a U-tube type sensing device, or tube container. As pointed out in U.S. Pat. No. 3,370,471, U-tube type densitometers are known for their instability and problems. Also, U-tube type densitometers are sensitive to the flow rate. Higher flow rates can result in liquid turbulence and centrifuging of the liquid while lower flow rates can result in the suspended heavier particles settling out of the liquid and building up on the inside of the densitometers.
Another problem encountered with U-tube type devices and other known devices is the use of flexible couplings of flexible tubing to connect the measurement device to the instrument. In U.S. Pat. No. 4,285,239, it is stated: "Nuclear densitometers and weight measuring type densitometers suffer certain inherent shortcomings. Nuclear densitometers work rather poorly when the density of the slurry is not uniform throughout the crossection of the pipe. The prior art weight measuring type densitometers on the other hand, are, generally speaking, unable to accurately determine the true weight of a given pipe section because of the inherent structural problem that the pipe section must be physically coupled to the pipeline. Various types of flexible joints were used for this purpose in prior art," and such are discussed in U.S. Pat. Nos. 3,503,267; 3,320,791; 688,388; 2,039,997; 2,613,530; 2,669,118; 3,812,723; 3,143,887; 1,905,558; 3,004,544; 3,039,310; 3,044,302; 3,151,775; 3,187,584; 3,020,765; 3,225,603; 3,258,973; 3,320,791; 3,330,161; 3,431,785 and 3,473,368. The above mentioned prior art have inherent problems due to the use of flexible couplers. Much effort and expense have been devoted toward the development of an accurate and dependable liquid weight measuring device.
Another inherent problem is that of material build-up inside the instrument or weight-measuring device. To overcome a material build-up attempts have been made to build a self-cleaning instrument. U.S. Pat. No. 3,320,791 discloses a device that is self-cleaning. However, a U-tube type device is employed in one application and flexible couplers, or bellows, in another application. Due to the required insertion of a semirigid tube or rod into the system, if the build-up is too great it results in deformation of the rod and tube or precludes insertion of the rod at all. Also, if the tube is processing sandfilled or corrosive liquids the sides of the tubes will be worn and the cleaning rod will not clean the inner walls when inserted.
U.S. Pat. No. 2,311,312 and U.S. Pat. No. 2,708,360 clean the tube by means of a flushing action. However, both of these devices use a flexible coupling as previously discussed. Also, in many instances it is not possible to flush the liquid being measured with another liquid due to contamination of the liquid being measured. The chemical industry is most familiar with this problem such as flushing an acid-based liquid with water. Therefore, flushing type systems are not always desirable or practical. In U.S. Pat. Nos. 3,320,790, 2,311,312 and 2,708,360, it is necessary to interrupt the normal flow of the liquid in order to clean or flush the weight measured liquid. Also, high flow velocities cannot be used with some slurries or liquids as the flow will become turbulent resulting in erroneous measurements. To date, prior art densitometers have not been able to overcome the combined problems common to U-tubes, flexible couplings and maintaining self-cleanliness.
It is therefore an object of this invention to provide a liquid weight measurement instrument which will accurately measure the density of the liquid in, or flowing through, the instrument.
Another object of this invention is to provide such a weight measurement instrument that is self-cleaning and therefore not subject to erroneous measurements due to sedimentation or build-up in the instrument.
A further object of this invention is to provide a liquid weight measurement instrument that is self-cleaning during the normal operation of the system so that no interruption of the measurement results during the cleaning cycle.
A still further object is to provide an instrument that is sensitive, accurate and versatile yet rugged and inexpensive and easy to manufacture. Another object of this invention is to provide an instrument which will eliminate problems encountered with earlier densitometers of U-tube type design and densitometers using flexible couplings within the measurement device.